Plastic lens are lightweight and rich in toughness, and are easily colored. Examples of performance particularly required of plastic lenses include low specific gravity, high transparency, low yellowing, optical performance in terms of high refractive index and high Abbe's number, high heat resistance and high strength. A high refractive index makes it possible to reduce thickness of the lens, while a high Abbe's number reduces chromatic aberration of the lens. Numerous organic compounds having sulfur atoms and/or selenium atoms have been reported in recent years for the purpose of imparting a high refractive index and Abbe's number. In particular, it is known that polyepisulfide compounds having sulfur atoms demonstrate a favorable balance between refractive index and Abbe's number. Consequently, optical materials have been reported in which sulfur is introduced to a polyepisulfide compound to maintain refractive index and thiourethane is introduced for improving shock resistance (Patent Document 1).
However, optical materials in which sulfur and thiourethane are introduced into a polyepisulfide compound have the problems of foaming and heat generation at the time of their production. Consequently, a process for producing optical materials has been reported that consists of reacting a prepolymer of an episulfide compound and sulfur atoms with a prepolymer of a compound having an isocyanate group and a compound having a mercapto group in order to suppress these problems (Patent Document 2).